1. Field of the Invention
The present invention relates to secondary refining of molten steel, and particularly, to a method of effectively lowering impurities (sulphur, oxygen, nitrogen and carbon) in molten steel to ultra-low ranges using a RH vacuum degassing unit.
2. Description of the Prior Technology
In secondary refining of molten steel, there has been known a method of supplying a flux in a vacuum vessel of a RH vacuum degassing unit for refining under desulphurization, wherein the flux freely falls on the bath surface within the vacuum vessel. Accordingly, for improving reaction rate, the flux in the form of fine powder must be used. This brings about a large disadvantage that the added flux is sucked through the exhaust system before reaching the bath surface of the molten steel. To cope with the disadvantage of using the fine powder flux, there has been proposed a method of using a massive flux; however, use of a massive flux degrades the reaction efficiency.
Also, there has been proposed a method of promoting the reaction while circulating both the molten steel and the flux by injecting a desulphurizing flux into the molten steel directly under a riser using the so-called immersion lance in the RH vacuum degassing unit disclosed in "Material and Process"; Vol 1. 1, p. 1189 (1988). This known technology, however, has disadvantages in that the immersion lance is short in its service life and is difficult in its management, and further, it is difficult to accurately guide both the injected gas and the flux in the riser and hence to manage the operation.
Further, there has been known a desulphurizing refining technology such as that disclosed in Japanese Patent Laid-open No. sho 63-114918. In this technology, a nozzle is provided on the inner wall of a vacuum vessel of a RH vacuum degassing unit in such a manner as to be inclined at 30.degree.-50.degree. with respect to the horizontal direction, and the desulphurization is performed by injecting 1.7-4.0 kg/t of a flux to the steel bath surface within the vessel. This known technology, however, is disadvantageous in that, since the flux is charged in the direction inclined to the steel bath surface, the catching efficiency of the flux to the molten steel becomes poor and the effective desulphurization is obstructed by the influence of the oxidizing potential of the slag on the steel bath.
Also, there has been such a technology as disclosed in Japanese Patent Laid-open No. sho 53-92320, wherein molten steel is secondarily refined by injecting a powder flux on the steel bath within a RH vacuum vessel. However, this known technology is intended to lower the oxygen concentration in the molten steel, and does not refer to the composition of the slag in a ladle which is extremely important in the desulphurizing treatment. Therefore, it is entirely obscure whether or not the above technology is effective in a desulphurizing treatment which is the subject of the present invention.
Further, Japanese Patent Laid-open No. sho 58-9914 discloses a VOD process, wherein the desulphurization is performed by injecting a powder flux together with a carrier gas on the steel bath surface under the reduced pressure using a top-injecting lance. However, this known technology does not teach how the desulphurizing reaction is effected by oxidizing slag (ladle slag), which inevitably flows out upon tapping the molten steel from the primary refining furnace such as a converter to a ladle. Therefore, it is doubtful whether the above technology may be applicable for desulphurizing treatment in a RH vacuum degassing unit.
On the other hand, the melting of ultra-low carbon steel is commonly performed by the steps of decarburization and dephosphorization in the converter, and decarburization and deoxidation in a specified carbon concentration using a secondary refining unit such as an RH vacuum degassing unit or a DH unit. In the melting method of this type, it is important to rapidly perform the decarburization and deoxidation to the low concentration range, which is also desirable for improving the quality of the steel and for preventing the surface defects due to non-metallic inclusions.
To meet the above demand, there has been proposed technologies of effectively performing deoxidation. For example, "Iron and Steel"; No. 11, Vol. 76, pp. 1932-1939 discloses a technology of preventing re-oxidation of the steel bath due to oxides (iron oxide or manganese oxide) in the converter slag floating on the steel bath in the ladle through reduction of the converter slag. However, in this technology, it is impossible to rapidly measure the amount and the composition of the converter slag floating on the steel bath in the ladle, and accordingly, the reduction is made unstable. For example, when a reducing agent is excessively charged, it reacts with the dissolved oxygen in the molten steel, which brings about the lack of the oxygen amount required for decarburization, or which causes the rephosphorization accompanied with the slag reducing action.
Further, it has been pointed out that the essential decarburization is occasionally stagnated, particularly, in the ultra-low carbon range (for example, as disclosed in "Material and Process"; No. 1, Vol. 1. 3, pp. 168 to 171).
As described above, in the conventional technologies, it is not considered how to control the composition of the primary refining slag (ladle slag) discharged from the converter and the composition of the secondary refining slag produced in the ladle or in the vacuum vessel of the RH vacuum degassing unit, which makes it impossible to perform the effective desulphurization and deoxidation.
For example, the above conventional technologies disclosed in Japanese Patent Laid-open Nos. sho 53-92320 and sho 63-114918 disclose the injection of the desulphurizing and deoxidizing flux; however, they do not refer to the composition of the slag in the ladle at all. On the other hand, in the technology proposed in Japanese Patent Laid-open No. sho 58-9914, there appears the description on such a slag composition. The description, however, is made not on the operation of the RH vacuum degassing unit, but on the VOD process in which the slag is strongly stirred together with the steel bath. Further, the proposal relates to the technology of adjusting the basicity of the slag, and thus is not applicable for the RH vacuum degassing treatment.
Also, in addition to the problems of the conventional technologies, the melting of ultra-low sulphur steel has generally the following problem: namely, in the case of performing the desulphurization up to the ultra-low sulphur concentration region, it is necessary to increase the injected amount and the injecting time of the powder flux, and accordingly, the temperature drop due to the powder flux must be compensated by increasing the temperature of the molten steel. However, if the furnace tapping temperature is increased, the life of the refractories in the converter is deteriorated. Needless to say, a method of performing desulphurization while compensating the temperature in the RH vacuum degassing treatment has been sought; but it has not been established as yet.
Further, in the case that the desulphurization is performed by injecting a powder flux on the surface of the molten steel in the RH vacuum degassing unit, it is desirable that the powder is circulated between the vacuum vessel and the ladle together with the flow of the molten steel and is finally caught in the ladle. The powder, however, is commonly in the state of floating on the steel bath surface within the vacuum vessel and is not circulated. Conventional technologies have not solved this problem as yet.